1. Field of the Invention
The present invention generally relates to an image transfer system, and in particular, to a system for transferring a toner image formed on an imaging member to a transfer medium for use in an imaging machine, such as an electrophotographic copier, facsimile machine or laser beam printer.
2. Description of the Prior Art
An electrophotographic imaging machine, such as an electrophotographic copier, is well known in the art. Such an electrophotographic imaging machine includes a photosensitive member typically comprised of a base of electrically conductive material and a photoconductive layer formed on the base. The photosensitive member is first charged uniformly by a corona charger, and then the thus charged photosensitive member is exposed to an original light image thereby forming an electrostatic latent image by having the uniform charge selectively dissipated in accordance with the original light image. The latent image thus formed is then developed, typically by toner, so that there is formed a powder visual image on the photosensitive member. Then, the powder image is transferred to a transfer medium, typically plain paper, by means of a transfer corona unit. In this case, the transfer medium is brought into contact with the surface of the photosensitive member on which the powder image is formed, and corona ions opposite in polarity to the toner powder are applied to the back side of the transfer medium thereby causing the powder image to be electrostatically attracted to the transfer medium from the photosensitive member. The transfer medium now bearing thereon the thus transferred image is typically subjected to an image fixing operation, for example, by application of heat to have the transferred image fixed to the transfer medium. On the other hand, after image transfer, the photosensitive member is cleaned to remove the residual toner thereby setting ready for the next cycle of operation.
In such an electrophotographic imaging machine, the electrically conductive base of the photosensitive member is grounded, and, thus, when the corona ions of a predetermined polarity are applied to the exposed surface of the photoconductive layer, charges of opposite polarity are induced in the base to maintain the charge neutrality. Thus, when corona ions are applied at the image transfer station to the back side of the transfer medium in contact with the photosensitive member, charges of opposite polarity are also induced in the base of the photosensitive member. In such a corona image transfer system, the transfer efficiency of toner powder fluctuates depending on the humidity, or the amount of water contents in the transfer medium. That is, if the humidity is high, the transfer medium contains more water so that its electrical resistance is effectively decreased, and, thus, the charges applied to the back side of the transfer medium from a corona unit will pass more through the transfer medium thereby leaking to the side of the photosensitive member. This will cause a deterioration in transfer efficiency and thus in the quality of transferred image.